A. Field of Invention
This invention pertains to a process for manufacture of optically active 2-(acyloxymethyl)-1,3-oxathiolanes of Formula I. Such products are useful intermediates in the manufacture of Apricitabine and related compounds.

B. Description of the Related Art
The 1,3-oxathiolane nucleosides comprise a clinically-important class of drugs for the treatment of human immunodeficiency virus (HIV), the etiologic agent responsible for the development of acquired immune deficiency syndrome in humans. Unlike the antecedant antivirals Abacavir, Didanosine, Stavudine, Tenofovir Disoproxil, Zalcitabine and Zidovudine, a 1,3-oxathiolane nucleoside hosts a pentacyclic thioacetal in place of a deoxyribose moiety.

Representative of the 1,3-oxathiolane nucleosides is Lamivudine, an HIV reverse transcriptase inhibitor that is clinically indicated in combination therapy with Zidovudine, or with Abacavir and Zidovudine. The pyrimidine base of Lamivudine is attached in a cis relationship to the carbon atom at position 5 of the 1,3-oxathiolane ring. Lamivudine, which is marketed as a single enantiomer of (−)-2(R)-cis absolute configuration, displays pronouncedly enhanced antiviral properties relative its mirror image isomer.

Emtricitabine is a second representative of the 1,3-oxathiolane nucleosides that was developed as a treatment regimen for HIV infection. This drug is dispensed as a fixed-dose combination with Efavirenz, Emtricitabine and Tenofovir Disoproxil. Emtricitabine is a fluorinated analogue of Lamivudine with a (−)-2(R)-cis absolute configuration. The racemic form of Emtricitabine, known as FTC, is also under clinical development in combination with other approved anti-HIV drugs.

Although the above nucleosides are constructive therapeutic regimens against HIV infection, many patients develop strains that become resistant to these drugs after prolonged periods of treatment. The sustained development of new anti-HIV drugs, therefore, remains a pivotal focus of the pharmaceutical industry. In this light, Apricitabine is currently undergoing clinical development and is showing very promising indications. Apricitabine has a mechanism of action similar to other nucleoside reverse transcriptase inhibitors making up the first-line therapies for treating HIV infection, but clinically this compound has exhibited the added benefits of a stronger safety profile, a broader activity profile against the difficult-to-treat, drug-resistant viruses, and a lower tendency to cause resistance in patients after prolonged treatment. Apricitabine has a 2(R)-cis absolute configuration, but in marked contrast to Lamivudine and Emtricitabine, the pyrimidine base in Apricitabine is attached to the 4-position of the 1,3-oxathiolane ring. Apricitabine is the first in class of a series of positional-switch 1,3-oxathiolane nucleosides that portray enhanced, broader spectrum anti-HIV activity.

Prior art teaches three processes for the preparation of 4-nucleobase-1,3-oxathiolanes. Common to each of these processes is a reaction of a purine or pyrimidine base with a compound of Formula II wherein R2 may be a hydrogen or an acyl group, and wherein X may be a leaving group, a displaceable atom or an NH2 group. Of consequence to the present invention may be optically pure 2-(acyloxymethyl)-1,3-oxathiolanes of Formula I. A Pummerer rearrangement of the latter may provide a compound of Formula II wherein R2 may be an acyl group and X may be a carboxylate leaving group. As a result, optically pure compounds of Formula I may serve as valuable intermediates for manufacture of Apricitabine and other 4-nucleobase-1,3-oxathiolane antiviral derivatives.

2-(Acyloxymethyl)-1,3-oxathiolanes of Formula I may be prepared by oxidative cleavage of glycerol monoesters. The product of this reaction may be a glycolaldehyde ester, which when treated with 2-mercaptoethanol may generate a 2-(acyloxymethyl)-1,3-oxathiolane. This method may find restricted commercial viability because the acylation of glycerol may be non-selective and may yield a mixture of the primary and secondary mono-esters unless glycerol is first converted to solketal. Furthermore, glycolaldehyde esters tend to be unstable compounds, which may restrict their purification, storage and handling.
Additionally, glycolaldehyde esters may be prepared by (i) condensation of an acyl chloride with glycolaldehyde diethyl acetal, (ii) ozonolytic cleavage of 2-butene-1,4-diol diesters, and (iii) oxidation of a 2-(hydroxyethyl)carboxylates. The major limitations facing these processes may include (i) a low yield in the oxidation of 2-(hydroxyethyl)carboxylates, (ii) the instability of glycolaldehyde esters, and (iii) the specialized and costly facility required for conducting an ozonolysis at commercial scale.
In addition, a racemic compound of Formula I wherein R1 is a phenyl group may be prepared in three steps from an alkali metal benzoate and a haloacetaldehyde acetal. In one such example, hydrolytic cleavage of 2-(benzoyloxy)acetaldehyde diethyl acetal may provide benzoyl glycolaldehyde and thence (+/−)-2-(benzoyloxymethyl)-1,3-oxathiolane upon acetalation with 2-mercaptoethanol. A major limitation of this process may be the instability of benzoyl glycolaldehyde. In an improvement upon this process, 2-(benzoyloxy)acetaldehyde diethyl acetal may be reacted with 2-mercaptoethanol directly to yield the 1,3-oxathiolane.
Prior art further teaches optically active, 1,3-oxathiolane nucleosides may be obtained by various techniques and methods including (i) asymmetric synthesis, (ii) chiral chromatography of a racemate, (iii) enzyme-mediated enantioselective catabolism, degradation or kinetic resolution, (iv) synthesis and separation of diastereomers derived from a chiral auxiliary, and (v) crystallization of diastereomeric salts.
An asymmetric synthesis of optically active, 5-nucleobase-1,3-oxathiolanes was reported wherein L-gulose may serve as the source of naturally occurring chirality. While interesting, this process may be restricted in scope and not easily extended to the production of optically active 4-nucleobase-1,3-oxathiolanes.
The separation of enantiomers by chiral chromatography may provide another avenue to optically active antiviral nucleosides. The compound of interest, or a convenient process intermediate such as a 2-(acyloxymethyl)-1,3-oxathiolane of Formula I, may be obtained in optically pure form by separation of the enantiomers on a suitably functionalized β-cyclodextrin, cellulose or amylose chromatographic medium using high performance liquid chromatography (HPLC) or simulated moving bed chromatography. While such processes may be scaled to commercial level, their utility may be constrained by the high cost of chiral chromatography relative to other methods of separating enantiomers, and by the lack of a recycle of the opposite enantiomer.
In addition, certain enzymes may be applied to resolutions of nucleoside derivatives. Emtricitabine may be obtained by a selective enzymatic degradation of a (+)-5′-monophosphate derivative with 5′-nucleotidase followed by alkaline phosphatase hydrolysis of (−)-5′-Emtricitabine phosphate. Enzyme-mediated enantioselective catabolism with cytidine-deoxycytidine deaminase may be utilized in a resolution of FTC. An immobilized cytidine-deoxycytidine deaminase enzyme was subsequently developed for this process. Additionally, enzymatic kinetic resolution of 5′-nucleoside carboxylates may be used in the synthesis of Emtricitabine. Pig liver esterase (PLE) may be applied to a resolution of FTC, and this process was scaled to the one hundred gram level. PLE may also be employed to resolutions of other 5′-nucleoside carboxylates. The resolution of 5′-FTC-carboxylates was further improved to yield Emtricitabine by employing an immobilized cholesterol esterase from Candida cyclindracea. The optical purity of Emtricitabine obtained by this process may be further enriched by recrystallization. The foremost limitation with each of the above enzymatic processes may be that the resolution step comes at or near the end of the synthetic pathway and the opposite enantiomer may be destroyed or not be easily racemized. With the aim of improving the economics of manufacture, one skilled in the art may prefer to conduct a resolution early in the manufacturing process. In this light, 2-(benzoyloxymethyl)-1,3-oxathiolane-4-propionate was screened against nine lipases and two proteases. The highest enantioselectivity (76%) and yield (14%) for the (−)-isomer was obtained with Mucor miehei lipase. The low yield and modest enantioselectivity of this process, unfortunately, may present significant impediments to its commercialization.
Moreover, optically pure isomers of compounds of Formulas I and II may be obtained from synthetic diastereomers incorporating (+)- and (−)-menthol as a chiral auxiliary. For example, the diastereomers of (1′R′,2′S,5′R)-menthyl-1,3-oxathiolane-2-carboxylate may be prepared and separated by fractional crystallization, and a subsequent reduction of the individual diastereomers may afford optically pure 2(R)- or 2(S)-(hydroxymethyl)-1,3-oxathiolanes. Enantiomers of certain nucleosides may also be separated via chiral auxiliaries. Whilst these methods may be practical at research scale, they may find limited utility for large scale manufacture because (i) the chiral auxiliary may significantly add to the cost, and (ii) the process may not render a recycle of the opposite enantiomer.
Furthermore, certain 4-nucleobase-1,3-oxathiolane racemates may be resolved by diastereomeric salt formation and fractional crystallization. For example, cis-(−)-2-(hydroxymethyl)-4-(cytosine-1′-yl)-1,3-oxathiolane may be resolved by fractional crystallization of a salt formed between the racemate and (1R)-(−)-10-camphorsulphonic acid. The principle drawback of this process may be the resolution occurs at or near the final step of the synthesis wherein racemization and recycle of the opposite enantiomer may not be possible.
The present invention describes a new and superior process for manufacture of optically pure 2-(acyloxymethyl)-1,3-oxathiolanes of Formula I. Such compounds may be useful intermediates in the manufacture of optically active antiviral drugs such as Apricitabine.